The present invention relates to a developing device for developing by using a developer an electrostatic latent image formed on an image carrier, more specifically to a developing device for developing the electrostatic latent image by using a magnetic developer.
A copying apparatus using a one-component magnetic developer (hereinafter referred to as magnetic toner) includes a developing device in which an electrostatic latent image formed on the surface of an image carrier (hereinafter referred to as photosensitive body), such as a selenium drum, is supplied with the magnetic toner to be developed.
Conventionally, the developing device includes supply means (hereinafter referred to as developing roller) which is composed of a hollow cylindrical rotating body formed of a nonmagnetic material such as aluminum, and a magnet roller to rotate inside the rotating body in the opposite direction to the rotating direction of the rotating body, for uniform development. FIG. 1 shows a conventional example of the developing device of this type.
In FIG. 1, reference numeral 1 designates a body frame of the developing device. The body frame 1 includes a first casing 2 having a doctor blade 2a, a second casing 3 opposed to the first casing 2 with a space between them, and a pair of side frames 4 to which both end portions of the first and second casings 2 and 3 are fixed severally. The body frame 1 has an opening 1a for supply at the upper side thereof and an opening 1b for development at one side thereof. A developing roller 5 is rotatably disposed inside the body frame 1. Part of the developing roller 5 is exposed to the outside through the developing opening 1b. Magnetic toner supplied through the supply opening 1a is stored at that portion of the inside space of the body frame 1 which is above the developing roller 5. The end portion of the second casing 3 on the side of the developing opening 1b is defined as a toner receiving section 3a.
The developing roller 5 includes a cylindrical rotating body (hereinafter referred to as sleeve) 5a made of a nonmagnetic material such as aluminum, and a magnet roller 5b disposed inside the sleeve 5a. The sleeve 5a is rotated by suitable means in the counterclockwise direction indicated by an arrow A in FIG. 1, and the magnet roller 5b rotates in the clockwise direction indicated by an arrow B in synchronism with the rotation of the sleeve 5a. A narrow gap 7 of a given width is defined between the outer peripheral surface of the sleeve 5a and a photosensitive drum 6 to rotate in the clockwise direction indicated by an arrow C in FIG. 1.
In such a developing device, the magnetic toner is adsorbed on the outer peripheral surface of the sleeve 5a by the magnetic force of the magnet roller 5b, and carried in the direction indicated by the arrow A. At this time, the magnet roller 5b is rotating in the opposite direction to the rotating direction of the sleeve 5a, so that the magnetic toner adsorbed on the sleeve 5a take the form of soft brushes standing substantially upright on the outer peripheral surface of the sleeve 5a. Then, the magnetic toner carried in this manner is attracted through the gap 7 to an electrostatic latent image formed on the surface of the photosensitive drum 6, in accordance with the surface potential of the image, and thus the image is developed satisfactorily.
The development does not require the use of all the magnetic toner on the sleeve 5a. That portion of the magnetic toner which remains on the sleeve 5a without having played a part in the development collected again in the body frame 1 by suitable means. A small amount of magnetic toner would, however, fall on to the toner receiving section 3a of the second casing 3 to accumulate therein.
In this prior art developing device, however, the magnet roller 5b rotates in the direction indicated by the arrow B, so that the magnetic toner accumulated in the toner receiving section 3a is moved toward the developing opening 1b and delivered to the outside of the second casing 3. Thereafter, the magnetic toner tends to drop and scatter as it moves away from the magnetic force of the magnet roller 5b. The scattered toner would soil peripheral devices to deteriorate the quality of copied pictures.